The combination of the new radiolabeled antibody agents and single photon emission computed tomographic (SPECT) imaging promises to significantly improve tumor detection and the delineation of disease state. SPECT quantitation with these radiopharmaceuticals will be useful for computing radionuclide therapeutic doses, monitoring therapy, and obtaining in vivo kinetic data. The clinical achievement of these goals is, however, limited by the image quality obtained when imaging the medium to high energy photon emitting radionuclides with which these agents have been labeled. The goal of this grant is to develop and test methods of three-dimensional (3-D) digital image restoration to be employed post-reconstruction which will reduce the effects of septal penetration, scatter, and spatial resolution losses while suppressing noise; all in a balanced manner. Three-D post- reconstruction filtering will be the focus of the proposed investigation because the use of a single or limited number of transfer functions should be a good approximation to use when deconvolving the images. The dependence of the transfer functions on phantom size and shape, and position within a given phantom will first be ascertained for In-111 and Ga-67. These transfer functions will then be used in the implementation and optimization of stationary and nonstationary versions of the constrained least-squares (CLS) and Metz filters. The Metz filter investigated will be our new image-dependent formulation. The optimization criteria employed will be the normalized mean square error as calculated both with and without gradient prefiltering to enhance the recovery of edges. An initial test of the performance of these filters according to standard measures of spatial resolution, image contrast, noise level, and processing time will be conducted to select those deemed suitable for further study. The selected filters will then be tested to determine their utility in facilitating tumor detection, volume estimation, and quantitation of activity. Tumor detection will be assessed via receiver operating characteristics (ROC) methods using SPECT images of the Alderson Organ Scanning Phantom. A study of volume quantitation in images of extended organs, and spheres covering a range of sizes will be conducted. A modified post- reconstruction attenuation correction technique will be developed and used to compare the quantitation of activity in terms of the linearity of quantitation, recovery coefficients, and the cross-talk index.